1. Field of the Invention
The present invention relates to an optical recording and reproducing apparatus having a plurality of light spots which can repetitively record and reproduce a signal by light irradiation.
2. Description of the Related Art
A prior art rewritable optical recording and reproducing apparatus having a plurality of light spots is shown in FIG. 15.
Numeral 3 denotes an information track on a recording medium, numeral 7 denotes a recording light source having a wavelength .lambda..sub.1, numeral 13 denotes an erasing light source having a wavelength .lambda..sub.2, numerals 8 and 14 denote collimating lenses, numerals 9 and 16 denote wavelength separation filters which transmit light of wavelength .lambda..sub.1 and reflect light of wavelength .lambda..sub.2, numeral 10 denotes a polarization beam splitter, numeral 11 denotes a .lambda..sub.1 /4 plate, numeral 15 denotes .lambda..sub.2 /4, numeral 12 denotes an objective lens, and numerals 18 and 19 denote photo-detectors. A light emitted from the recording light source 7 advances as shown by a solid line and forms a recording light spot L on the information track 3 to record and reproduce information, and a reflected light therefrom is directed to the photo-detector 18 which generates a focus error signal (FE.sub.L) and a tracking error signal (TE.sub.L). A light emitted from the erasing light source 13 advances as shown by a broken line and forms an erasing light spot M on the track 3 to erase the information. Most portions of the reflected light therefrom is directed to the photo-detector 19 which generates a tracking error signal (TE.sub.M) of the erasing light spot M.
An optical system is arranged such that the light spots L and M are on a line which is essentially parallel to the track 3. An objective lens drive element 20 is arranged on the objective lens 12 so that known focus control and tracking control are effected for the recording light spot L by the signals FE.sub.L and TE.sub.L. In the control, the erasing light spot M is also controlled in the same manner and moved by the same distance as the recording light spot. A rotatable mirror 22 is arranged in a light path of the erasing light spot M so that the erasing light spot M is independently movable in a direction normal to the track 3. The tracking error signal TE.sub.M of the erasing light spot M derived from the photo-detector 19 through a differential amplifier 24 is supplied to a driver circuit 26 through a phase compensation circuit 25, and supplied to a driver 23 to drive the rotatable mirror 22 to independently control tracking of the erasing light spot M.
In the above arrangement, after the recording light spot L has been scanned, the erasing light spot M is scanned so that the two spots L and M can be tracked to one track. In this manner, a relative positional deviation of the two light spots L and M can be prevented.
The drive means for moving the light spot M normal to the track may be implemented by moving the erasing light source 13 by a piezoelectric element or arranging a rotatable prism in the light path.
However, in the above arrangement, if the light spot deviates from the center of the target track currently being scanned by more than 1/2 track pitch normal to the track when an external disturbance is applied, a tracking error signal is no longer an error signal for the target object and a track other than the target track is scanned. As a result, a track other than the target track may be miserased, misrecorded or misreproduced.